Color Holographic Display System

ABSTRACT

A display system comprising a first plurality of pixels, a second plurality of pixels, a first Fourier transform lens and a second Fourier transform lens. The first plurality of pixels is arranged ranged to display first holographic data corresponding to a first holographic reconstruction and receive light of a first wavelength. The a second plurality of pixels is arranged to display second holographic data corresponding to a second holographic reconstruction and receive light of a second wavelength. The first Fourier transform lens is arranged to receive spatially modulated light having a first wavelength from the first plurality of pixels and perform an optical Fourier transform of the received light to form the first holographic reconstruction at a replay plane, wherein the first holographic reconstruction is formed of light at the first wavelength. The second Fourier transform lens is arranged to receive spatially modulated light having a second wavelength from the second plurality of pixels and perform an optical Fourier transform of the received light to form the second holographic reconstruction at the replay plane, wherein the second holographic reconstruction is formed of light at the second wavelength. The optical path length from the first Fourier transform lens to the replay plane is not equal to the optical path length from the second Fourier transform lens to the replay plane.

FIELD

The present disclosure relates a display system. More specifically, thepresent disclosure relates to a colour display system. Yet morespecifically, the present disclosure relates to a colour holographicprojector. Embodiments relate to a head-up display and near-eye device.

Introduction and Background

Light scattered from an object contains both amplitude and phaseinformation. This amplitude and phase information can be captured on,for example, a photosensitive plate by well-known interferencetechniques to form a holographic recording, or “hologram”, comprisinginterference fringes. The “hologram” may be reconstructed byilluminating it with suitable light to form a holographicreconstruction, or replay image, representative of the original object.

It has been found that a holographic reconstruction of acceptablequality can be formed from a “hologram” containing only phaseinformation related to the original object. Such holographic recordingsmay be referred to as phase-only holograms. Computer-generatedholography may numerically simulate the interference process, usingFourier techniques for example, to produce a computer-generatedphase-only hologram. A computer-generated phase-only hologram may beused to produce a holographic reconstruction representative of anobject.

The term “hologram” therefore relates to the recording which containsinformation about the object and which can be used to form areconstruction representative of the object. The hologram may containinformation about the object in the frequency, or Fourier, domain.

A computer-generated phase-only hologram may be “pixelated”. That is,the phase-only hologram may be represented on an array of discrete phaseelements. Each discrete element may be referred to as a “pixel”. Eachpixel may act as a light modulating element such as a phase modulatingelement. A computer-generated phase-only hologram may therefore berepresented on an array of phase modulating elements such as a liquidcrystal spatial light modulator (SLM). The SLM may be reflective meaningthat modulated light is output from the SLM in reflection.

Each phase modulating element, or pixel, may vary in state to provide acontrollable phase delay to light incident on that phase modulatingelement. An array of phase modulating elements, such as a Liquid CrystalOn Silicon (LCOS) SLM, may therefore represent (or “display”) acomputationally-determined phase-delay distribution. If the lightincident on the array of phase modulating elements is coherent, thelight will be modulated with the holographic information, or hologram.The holographic information may be in the frequency, or Fourier, domain.Alternatively, the phase-delay distribution may be recorded on akinoform. The word “kinoform” may be used generically to refer to aphase-only holographic recording, or hologram.

The phase delay may be quantised. That is, each pixel may be set at oneof a discrete number of phase levels.

The phase-delay distribution may be applied to an incident light wave(by illuminating the LCOS SLM, for example) and reconstructed. Theposition of the reconstruction in space may be controlled by using anoptical Fourier transform lens, to form the holographic reconstruction,or “image”, in the spatial domain. Alternatively, no Fourier transformlens may be needed if the reconstruction takes place in the far-field.

A computer-generated hologram may be calculated in a number of ways,including using algorithms such as Gerchberg-Saxton. TheGerchberg-Saxton algorithm may be used to derive phase information inthe Fourier domain from amplitude information in the spatial domain(such as a 2D image). That is, phase information related to the objectmay be “retrieved” from intensity, or amplitude, only information in thespatial domain. Accordingly, a phase-only holographic representation ofan object may be calculated.

The holographic reconstruction may be formed by illuminating the Fourierdomain hologram and performing an optical Fourier transform, using aFourier transform lens, for example, to form an image (holographicreconstruction) at a reply field such as on a screen.

FIG. 1 shows an example of using a reflective SLM, such as a LCOS-SLM,to produce a holographic reconstruction at a replay field location, inaccordance with the present disclosure.

A light source (110), for example a laser or laser diode, is disposed toilluminate the SLM (140) via a collimating lens (111). The collimatinglens causes a generally planar wavefront of light to become incident onthe SLM. The direction of the wavefront is slightly off-normal (e.g. twoor three degrees away from being truly orthogonal to the plane of thetransparent layer). The arrangement is such that light from the lightsource is reflected off a mirrored rear surface of the SLM and interactswith a phase-modulating layer to form an exiting wavefront (112). Theexiting wavefront (112) is applied to optics including a Fouriertransform lens (120), having its focus at a screen (125).

The Fourier transform lens (120) receives a beam of phase-modulatedlight exiting from the SLM and performs a frequency-space transformationto produce a holographic reconstruction at the screen (125) in thespatial domain.

In this process, the light-in the case of an image projection system,the visible light—from the light source is distributed across the SLM(140), and across the phase modulating layer (i.e. the array of phasemodulating elements). Light exiting the phase-modulating layer may bedistributed across the replay field. Each pixel of the hologramcontributes to the replay image as a whole. That is, there is not aone-to-one correlation between specific points on the replay image andspecific phase-modulating elements.

The Gerchberg Saxton algorithm considers the phase retrieval problemwhen intensity cross-sections of a light beam, I_(A)(x,y) andI_(B)(x,y), in the planes A and B respectively, are known and I_(A)(x,y)and I_(B)(x,y) are related by a single Fourier transform. With the givenintensity cross-sections, an approximation to the phase distribution inthe planes A and B, Φ_(A)(x,y) and Φ_(B)(x,y) respectively, is found.The Gerchberg-Saxton algorithm finds solutions to this problem byfollowing an iterative process.

The Gerchberg-Saxton algorithm iteratively applies spatial and spectralconstraints while repeatedly transferring a data set (amplitude andphase), representative of I_(A)(x,y) and I_(B)(x,y), between the spatialdomain and the Fourier (spectral) domain. The spatial and spectralconstraints are I_(A)(x,y) and I_(B)(x,y) respectively. The constraintsin either the spatial or spectral domain are imposed upon the amplitudeof the data set. The corresponding phase information is retrievedthrough a series of iterations.

A holographic projector may be provided using such technology. Suchprojectors have found application in head-up displays for vehicles andnear-eye devices, for example.

A colour 2D holographic reconstruction can be produced and there are twomain methods of achieving this. One of these methods is known as“frame-sequential colour” (FSC). In an FSC system, three lasers are used(red, green and blue) and each laser is fired in succession at the SLMto produce each frame of the video. The colours are cycled (red, green,blue, red, green, blue, etc.) at a fast enough rate such that a humanviewer sees a polychromatic image from a combination of the threelasers. Each hologram is therefore colour specific. For example, in avideo at 25 frames per second, the first frame would be produced byfiring the red laser for 1/75^(th) of a second, then the green laserwould be fired for 1/75^(th) of a second, and finally the blue laserwould be fired for 1/75^(th) of a second. The next frame would then beproduced, starting with the red laser, and so on.

Another alternative method, that will be referred to as “spatiallyseparated colours” (SSC) involves all three lasers being fired at thesame time, but taking different optical paths, e.g. each using adifferent SLM or different spatial areas on the same SLM, and thencombining to form the colour image.

An advantage of the SSC (spatially separated colours) method is that theimage is brighter due to all three lasers being fired at the same time.However, if due to space limitations it is required to use only one SLM,the surface area of the SLM can be divided into three equal parts,acting in effect as three separate SLMs. The drawback of this is thatthe quality of each single-colour image is decreased, due to thedecrease of SLM surface area available for each monochromatic image. Thequality of the polychromatic image is therefore decreased accordingly.The decrease of SLM surface area available means that fewer pixels onthe SLM can be used, thus reducing the quality of the image.

Holographic colour display systems suffer from two significant problems.Firstly, a mismatch between the physical size of the different colourholographic reconstructions. Secondly, the composite colour image is oflow quality because of a resolution mismatch between the differentcolour holographic reconstructions.

The present disclosure addresses at least these problems.

SUMMARY

Aspects of an invention are defined in the appended independent claims.

There is provided a full colour display system comprising an opticalsystem and a processing system, the optical system comprising: one ormore spatial light modulators arranged to display holographic data inthe Fourier domain; multiple light sources arranged to illuminate thespatial light modulator(s); a viewing system arranged to produce avirtual image of the 2D holographic reconstruction; and wherein theprocessing system is arranged to: combine Fourier domain datarepresentative of a 2D image with Fourier domain data representative ofa phase only lens for each colour to produce first holographic data, andprovide the first holographic data to the optical system to produce avirtual image.

The Fourier domain data representative of each 2D colour image may becombined with Fourier domain data representative of a phase only lens;wherein the focal length of the phase only lens is inverselyproportional to the wavelength of the colour.

The zero-order block may be formed on a dichroic mirror.

The spatial light modulator may be a reflective LCOS spatial lightmodulator.

The optical power of the phase only lens may be user controlled.

The display system may comprise a near-eye display.

The display may be part of a HUD.

The replay fields may be spatially remote from the viewer.

The display system may further comprise a spatial filter configured toselectively block at least one diffraction order of the 2D holographicreconstruction and, optionally, the zero order.

The display system may include a Fourier transform lens and a zero orderblock arranged to produce a 2D holographic reconstruction in the spatialdomain corresponding to the holographic data.

The virtual images may be sequential frames of a 2D video.

The pixellated array may consist of pixels having a diameter less than15 μm.

There is also provided a method of displaying images comprising:combining holographic image data and lensing data to a number of spatiallight modulators; illuminating the spatial light modulators, each with adifferent collimated laser beam; applying the resultant light to anoptical system for forming a virtual image; and reconstructing, byFourier transformation, the individual colour images at the same planeto form a colour replay field where each single colour image has thesame size.

The method may further comprise spatially filtering the resultant lightfrom the SLM to selectively block at least one diffraction order of the2D holographic reconstruction.

In some embodiments, the hologram is calculated using an algorithm basedon the Gerchberg-Saxton algorithm such as described in British patent2,498,170 or 2,501,112 which are hereby incorporated in their entiretyby reference. However, some embodiments relate to Fourier holography andGerchberg-Saxton type algorithms by way of example only. The presentdisclosure is equally applicable to Fresnel holography and hologramscalculated by other techniques such as those based on point cloudmethods.

The term “hologram” is used to refer to the recording which containsamplitude and/or phase information about the object. The term“holographic reconstruction” is used to refer to the opticalreconstruction of the object which is formed by illuminating thehologram. The term “replay field” is used to refer to the plane in spacewhere the holographic reconstruction is formed. The terms “image” and“image region” refer to areas of the replay field illuminated by lightforming the holographic reconstruction. In embodiments, the “image” maycomprise discrete spots which may be referred to as “image pixels”.

The terms “encoding”, “writing” or “addressing” are used to describe theprocess of providing the plurality of pixels of the SLM with a respectplurality of control values which respectively determine the modulationlevel of each pixel. It may be said that the pixels of the SLM areconfigured to “display” a light modulation distribution in response toreceiving the plurality of control values.

In some embodiments, the spatial light modulator is a phase-only spatiallight modulator. These embodiments are advantageous because no opticalenergy is lost by modulating amplitude. Accordingly, an efficientholographic projection system is provided. However, the presentdisclosure may be equally implemented on an amplitude-only spatial lightmodulator or an amplitude and phase modulator. It may be understood thatthe hologram will be correspondingly phase-only, amplitude-only orfully-complex.

The term “light” is used herein in its broadest sense. Some embodimentsare equally applicable to visible light, infrared light and ultravioletlight, and any combination thereof.

Reference is made to holographic data comprising an image component anda lensing component to reflect that the holographic data is formed bythe combination (such as vector addition) of first holographic datawhich corresponds to the image and second holographic data whichcollectively provide a lensing effect to received light. In someembodiments described herein, each holographic data is a 2D array ofdata values. The first holographic data may be said to correspond to theimage because it contains information sufficient to form—such asrecreate or reconstruct—the image. In some embodiments described herein,the first holographic data may be said to correspond to an image becauseit is a frequency (or Fourier) domain representation of the image. Thesecond holographic data may be said to collectively provide a lensingeffect to received light because its effect on received light is thesame as that of a physical lens. Examples are given in the detaileddescription of how this may be achieved. The second holographic data maybe computationally-determined (or “software-defined”) to provide anyconceivable lensing effect or function—such as positive optical power,negative optical power or aberration correction, for example. In someembodiments disclosed herein, the second holographic data functions as aFourier transform lens. That is, it manipulates received light in thesame way as an appropriately-positioned Fourier transform optic such asa Fourier transform lens. In some such embodiments, the secondholographic data perform an optical Fourier transform of the firstholographic data. In this respect, it may be understood that theholographic data comprises a first component responsible for providing afirst optical function (i.e. modulating light with data corresponding tothe image) and a second component responsible for providing a secondoptical function (i.e. a Fourier transform).

The term “software-defined” (or “software-controllable”) is used toreflect that the data is computational data or software data which maybe changed or varied—including changed or varied in real-time-usingsoftware running on a computer. In this respect, the data may beconsidered dynamically-variable or simply “dynamic”.

Some embodiments describe 1D and 2D holographic reconstructions by wayof example only. In other embodiments, the holographic reconstruction isa 3D holographic reconstruction. That is, in some embodiments, eachcomputer-generated hologram forms a 3D holographic reconstruction.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments, based on the technology described above, willfollow, by way of example only. It will be appreciated that routinevariations can be made to alter the specific details provided herein.The examples are described with reference to the accompanying drawings,in which:

FIG. 1 is a schematic showing a reflective SLM, such as a LCOS, arrangedto produce a holographic reconstruction at a replay field location;

FIG. 2 depicts a composite colour holographic reconstruction inaccordance with prior art;

FIG. 3 depicts a first embodiment; and

FIG. 4 depicts a second embodiment.

In the figures like reference numerals referred to like parts.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention is not restricted to the embodiments described inthe following but extends to the full scope of the appended claims. Thatis, the present invention may be embodied in different forms and shouldnot be construed as limited to the described embodiments, which are setout for the purpose of illustration.

Terms of a singular form may include plural forms unless specifiedotherwise.

A structure described as being formed at an upper portion/lower portionof another structure or on/under the other structure should be construedas including a case where the structures contact each other and,moreover, a case where a third structure is disposed there between.

In describing a time relationship—for example, when the temporal orderof events is described as “after”, “subsequent”, “next”, “before” orsuchlike—the present disclosure should be taken to include continuousand non-continuous events unless otherwise specified. For example, thedescription should be taken to include a case which is not continuousunless wording such as “just”, “immediate” or “direct” is used.

Although the terms “first”, “second”, etc. may be used herein todescribe various elements, these elements are not be limited by theseterms. These terms are only used to distinguish one element fromanother. For example, a first element could be termed a second element,and, similarly, a second element could be termed a first element,without departing from the scope of the appended claims.

Features of different embodiments may be partially or overall coupled toor combined with each other, and may be variously inter-operated witheach other. Some embodiments may be carried out independently from eachother, or may be carried out together in co-dependent relationship.

In overview, a spatial light modulator (SLM) forms an array ofphase-modulating elements that collectively form a phase-onlyrepresentation of a desired image which can be reconstructed bycorrectly illuminating the SLM, to form a projector. The phase-onlydistribution may be referred to as a hologram. A Fourier transform lensis used to transform the holographic recording, which represents theobject for reconstruction, into an “image”. The image may be describedas the holographic reconstruction. The individual modulating elements ofthe SLM may be referred to as pixels. In embodiments, composite colourholographic reconstructions are provided by combining a plurality ofmonochromatic holographic reconstructions.

Light is incident across the phase-modulating layer (i.e. the array ofphase modulating elements) of the SLM. Modulated light exiting thephase-modulating layer is distributed across the replay field. Notably,in the disclosed types of holography, each pixel of the hologramcontributes to the whole reconstruction. That is, there is not aone-to-one correlation between specific points on the replay field andspecific phase-modulating elements.

The position of the holographic reconstruction in space is determined bythe dioptric (focusing) power of the Fourier transform lens. In someembodiments, the Fourier transform lens is a physical lens as per theexample shown in FIG. 1. That is, the Fourier transform lens is anoptical Fourier transform lens and the Fourier transform is performedoptically. Any lens can act as a Fourier transform lens but theperformance of the lens will limit the accuracy of the Fourier transformit performs. The skilled person understands how to use a lens to performan optical Fourier transform. However, in other embodiments, the Fouriertransform is performed computationally by including lensing data in theholographic data. That is, the hologram includes data representative ofa lens as well as data representing the object.

It is known in the field of computer-generated hologram how to calculateholographic data representative of a lens. The holographic datarepresentative of a lens may be referred to as a software-defined lensor software lens. For example, a phase-only holographic lens may beformed by calculating the phase delay caused by each point of the lensowing to its refractive index and spatially-variant optical path length.For example, the optical path length at the centre of a convex lens isgreater than the optical path length at the edges of the lens. Anamplitude-only holographic lens may be formed by a Fresnel zone plate.It is also known in the art of computer-generated hologram how tocombine holographic data representative of a lens with holographic datarepresentative of the object so that a Fourier transform can beperformed without the need for a physical Fourier lens. In someembodiments, lensing data is combined with the holographic data bysimple vector addition. In some embodiments, a physical lens is used inconjunction with a software lens to perform the Fourier transform.Alternatively, in other embodiments, the Fourier transform lens isomitted altogether such that the holographic reconstruction takes placein the far-field. In further embodiments, the hologram may includegrating data—that is, data arranged to perform the function of a gratingsuch as beam steering. Again, it is known in the field ofcomputer-generated hologram how to calculate such holographic data andcombine it with holographic data representative of the object. Forexample, a phase-only holographic grating may be formed by modelling thephase delay caused by each point on the surface of a blazed grating. Anamplitude-only holographic grating may be simply superimposed on anamplitude-only hologram representative of an object to provide angularsteering of an amplitude-only hologram.

The so-called “Fourier path length” is the optical path length from theFourier transform lens to the corresponding holographic reconstruction.The size of the holographic reconstruction, I, is related to the Fourierpath length as follows:

$\begin{matrix}{{I\lbrack {x,y} \rbrack} = {2 \cdot f \cdot {\tan ( {\sin^{- 1}( \frac{\lambda}{2 \cdot {p\lbrack {x,y} \rbrack}} )} )}}} & (1)\end{matrix}$

where f is the Fourier path length, λ is the wavelength of the light andp is the size of the pixel on the SLM.

It may therefore be understood that for any given Fourier path lengthand spatial light modulator pixel size, differing hologramreconstruction sizes will result for different colours. For example:

-   -   f=300 mm    -   Red (λr)=640 nm    -   Green (λg)=532 nm    -   Blue (λb)=450 nm    -   p[x,y]=8 um

This gives rise to three different sized hologram reconstructions:

-   -   Red=24.02 mm,    -   Green=19.96 mm, and    -   Blue=16.88 mm.

In prior systems, since the Fourier path length is the same for all ofthe colour channels, the hologram reconstruction as shown in FIG. 2 willresult.

FIG. 2 depicts a red holographic reconstruction 201, a green holographicreconstruction 203 and a blue holographic reconstruction 205 co-axiallyaligned at a reply plane.

The largest full colour image (the area where red, green and blueoverlap) is limited by the size of the blue hologram reconstruction.This presents a further problem: it is desirable to calculate hologramreconstructions using a single computation engine. Ideally the sameresolution would be calculated for each image, however, if this were thecase, the resolution of each colour of the full colour image would bedifferent. For example:

Hologram  Resolution  [X, Y] = 1024 × 1024.Full  Colour  Area : Blue  Resolution  [X, Y] = 1024.${{Full}\mspace{14mu} {Colour}\mspace{14mu} {{Area}:{{Green}\mspace{14mu} {{Resolution}\mspace{14mu}\lbrack {X,Y} \rbrack}}}} = {{{\frac{1024}{19.96\mspace{14mu} {mm}} \cdot 16.88}\mspace{14mu} {mm}} = 866}$${{Full}\mspace{14mu} {Colour}\mspace{14mu} {{Area}:{{Red}\mspace{14mu} {{Resolution}\mspace{14mu}\lbrack {X,Y} \rbrack}}}} = {{{\frac{1024}{24.02\mspace{14mu} {mm}} \cdot 16.88}\mspace{14mu} {mm}} = 720}$

It is undesirable for the individual wavelengths (colours) to havediffering resolutions within the full colour image area, as this leadsto a degradation in image quality.

A solution is to vary the resolution of the computed hologram to ensureall colours have the same resolution with the full colour area.

The resolution is determined as follow:

Full  Colour  Area : Blue  Resolution  [X, Y] = 1024.${{Full}\mspace{14mu} {Colour}\mspace{14mu} {{Area}:{{Green}\mspace{14mu} {{Resolution}\mspace{14mu}\lbrack {X,Y} \rbrack}}}} = {{{\frac{1024}{16.88\mspace{14mu} {mm}} \cdot 19.96}\mspace{14mu} {mm}} = 1210}$${{Full}\mspace{14mu} {Colour}\mspace{14mu} {{Area}:{{Red}\mspace{14mu} {{Resolution}\mspace{14mu}\lbrack {X,Y} \rbrack}}}} = {{{\frac{1024}{16.88\mspace{14mu} {mm}} \cdot 24.02}\mspace{14mu} {mm}} = 1458}$

However, computationally, this is undesirable because the blue channelis a power of 2 number (i.e. 2{circumflex over ( )}10=1024) and iscomputed efficiently using an FFT. The Red and Green channels are nolong power of 2 numbers and more importantly are significantly higherresolution. Given that the computation requirements of FFTs arelogarithmic, any increase in resolution is undesirable.

The inventors have recognised that it is advantageous to use differentFourier path lengths for each colour channel. The different Fourier pathlengths are determined by the use of Fourier lenses.

There is therefore provided a display system comprising: a firstplurality of pixels arranged to display first holographic datacorresponding to a first holographic reconstruction and receive light ofa first wavelength; a second plurality of pixels arranged to displaysecond holographic data corresponding to a second holographicreconstruction and receive light of a second wavelength; a first Fouriertransform lens arranged to receive spatially modulated light having afirst wavelength from the first plurality of pixels and perform anoptical Fourier transform of the received light to form the firstholographic reconstruction at a replay plane, wherein the firstholographic reconstruction is formed of light at the first wavelength; asecond Fourier transform lens arranged to receive spatially modulatedlight having a second wavelength from the second plurality of pixels andperform an optical Fourier transform of the received light to form thesecond holographic reconstruction at the replay plane, wherein thesecond holographic reconstruction is formed of light at the secondwavelength, wherein the optical path length from the first Fouriertransform lens to the replay plane is not equal to the optical pathlength from the second Fourier transform lens to the replay plane.

An embodiment is shown in FIG. 3 in which the respective Fourier pathlengths are different.

FIG. 3 shows three colour channels by way of example only. The presentdisclosure is equally applicable to any plurality of light channels Thefirst colour channel comprises a first SLM 301 arranged to receive bluelight 303 via a first beam splitter 305. The blue light is spatiallymodulated by the SLM 301. The phase-modulation provided by SLM 301comprises an image component and a Fourier lensing component. The SLM301 is reflected and the spatially-modulated light is directed to thereplay plane 350 by a mirror 307. Optionally, the mirror 307 comprisesan aperture to remove zero-order diffracted light from the SLM 310.Likewise, there is provided a green channel comprising a second SLM 311arranged to receive green light 313 via a second beam splitter 315. Thegreen light is spatially modulated by the second SLM 311. Thephase-modulation provided by second SLM 311 comprises an image componentand a Fourier lensing component. The second SLM 311 is reflected and thespatially-modulated light is directed to the replay plane 350 by asecond mirror 317. Optionally, the second mirror 317 comprises anaperture to remove zero-order diffracted light from the second SLM 311.Further likewise, there is provided a red channel comprising a third SLM321 arranged to receive red light 323 via a third beam splitter 325. Thered light is spatially modulated by the third SLM 321. Thephase-modulation provided by third SLM 321 comprises an image componentand a Fourier lensing component. The third SLM 321 is reflected and thespatially-modulated light is directed to the replay plane 350 by a thirdmirror 327. Optionally, the third mirror 327 comprises an aperture toremove zero-order diffracted light from the third SLM 321. In someembodiments, a light-receiving surface—such as a screen or diffuser—ispositioned at the replay field 350.

In FIG. 3, the first optical path length 309 from SLM 301 to the replayplane is greater than the second optical path length 319 from second SLM311 to the replay field which is in turn greater than the third opticalpath length 329 from third SLM 321 to the replay field. Each opticalpath length may be referred to as the “Fourier path length” for thatchannel because the Fourier lens is effectively on the SLM. It maytherefore be understood that the Fourier path length iswavelength-dependent, optionally, inversely proportional to wavelength.Each Fourier path comprises a respective mirror, wherein at least one ofthe respective mirrors is a dichroic mirror. In some embodiments, theplurality of mirrors and replay plane are disposed on a common opticalpath. In some embodiments, the plurality of mirrors and replay plane arecollinear.

The second mirror 317 is a first dichroic mirror which is substantiallyreflective to green light but substantially transmissive to blue light.The third mirror 327 is a second dichroic mirror which is substantiallyreflective to red light but substantially transmissive to green and bluelight. The person skilled in the art is familiar with the use ofdichroic coatings on mirrors to provide the functionality described.FIG. 3 describes blue, green and red channels by way of example only andthe present disclosure is applicable to any plurality of differentwavelength channels. It will be apparent that the first mirror 307 doesnot need to be a dichroic mirror.

In some embodiments, the device comprises two light channels and onedichroic mirror (or mirror with a dichroic coating). In otherembodiments, the device comprises three light channels and two differentdichroic mirrors (or mirrors with a dichroic coating). In someembodiments, the device comprises n light channels and (n−1) differentdichroic mirrors (or mirrors with a dichroic coating). Again, the personskilled in the art will know how to provide the necessary dichroicmirror/s or different dichroic coatings on mirror/s to achieve theoptical transmittance/reflectance described.

The use of (n−1) dichroic mirrors—where n is the number of lightchannels—allows the different colour channels to be directed onto acommon optical axis to the replay plane. In particular, the colourchannels are collinear in the region from the final mirror to the replayplane. The described use of at least one dichroic mirror thereforeprovides substantially collinear optical paths. Referring back to FIG. 3by way of example, the second mirror 317 directs the green light onto acommon optical path with the blue light and the third mirror 327 directsthe red light onto said common optical path with the blue and greenlight.

The display device therefore comprises a collinear optical path for theplurality of colour channels, wherein the colour channels have differentFourier path lengths.

This collinear optical path helps with optical alignment, stray lightmanagement and helps keep the device compact. Notably, the first mirror307 of FIG. 3 may be independently aligned—that is without affecting thegreen or red light paths. When the blue light is aligned at the replayfield using the first mirror 307, the second mirror 317 may be alignedwithout affecting (e.g. misaligning or moving out of alignment) the bluelight. Finally, the red light may then be aligned without affecting(e.g. misaligning or moving out of alignment) the blue or green light.There is therefore provided a device which is easier to align. There isalso provided an improved method of aligning a display device having aplurality of colour channels.

Another embodiment is shown in FIG. 4.

FIG. 4 is substantially identical to FIG. 3 but the beam splitters havebeen omitted for simplicity. Any number of different opticalconfiguration for illuminating the spatial light modulators may beconceived. FIG. 4 additionally shows a physical lens on each colourchannel which focuses zero-order (i.e. unmodulated) light received fromthe spatial light modulator through an aperture in the correspondingmirror 408/418/428 and out of the system. The modulated light from eachspatial light modulator may be focused to a different plane in space byadding optical power to the respective holographic data, for example.Accordingly, a method of removing the zero-order light from each colourchannel is provided without adversely affecting the holographicreconstruction. The arrows on the light rays in FIG. 4 merely illustratethe direction of illumination of each spatial light modulator and thedirection of reconstruction. For the avoidance of doubt, the zero-orderlight travels from left to right as shown in FIG. 3.

In more detail, a further embodiment is shown in FIG. 4 comprising ablue channel 403, a green channel 413 and a red channel 423. The bluechannel 403 comprises an SLM 401 and mirror 408 arranged to directspatially-modulated light from SLM 401 to the replay field 450. Thegreen channel 513 comprises a second SLM 411 and second mirror 418arranged to direct spatially-modulated light from the second SLM 411 tothe replay field 450. The red channel 423 comprises a third SLM 421 anda third mirror 428 arranged to direct spatially-modulated light from thethird SLM 421 to the replay field 450. In some embodiments, alight-receiving surface—such as a screen or diffuser—is positioned atthe replay field 450. Again, each mirror 401, 418 and 428 comprises acentral aperture arranged to remove zero-order diffracted light from thesystem. In FIG. 4, the distance 409 is greater than distance 419 whichis in turn greater than distance 429.

In particular, the inventors have recognised that by forming the fullcolour image in this manner, the number of pixels in the image for eachcolour is constant, thereby enabling a common computation engine to beused for all three colour channels. Conveniently, the opticalconfiguration, shown in FIG. 3, is advantageous from a packagingperspective as the need to mix colour channels requires dichroic mirrorsto be spatially displaced and this spatial displacement may be used aspart of the variable Fourier path lengths.

In embodiments, the wavelength-dependant path lengths may be determinedas follows:

$\begin{matrix}{{f(\lambda)} = \frac{I\lbrack {x,y} \rbrack}{2 \cdot {\tan ( {\sin^{- 1}( \frac{\lambda}{2 \cdot {p\lbrack {x,y} \rbrack}} )} )}}} & (2)\end{matrix}$

where f=Fourier path length, I=size of the holographic reconstruction,2=wavelength of each colour channel and p=size of the pixel on the SLM.

For example, the Fourier path length (Fourier lens focal length [FL])may be determined for each wavelength as follows:

-   -   I=30 mm    -   Red (λr)=640 nm    -   Green (λg)=532 nm    -   Blue (λb)=450 nm    -   p[x,y]=8 um

This gives rise to three different sized hologram reconstructions:

-   -   Red FL=374.7 mm    -   Green FL=450.9 mm    -   Blue FL=533.1 mm

In embodiments, there is therefore provided a full-colour virtual imagewhere the size of each individual colour image is the same. That is, itmay therefore be understood that, in embodiments, the first optical pathlength and second optical path length are such that the firstholographic reconstruction and second holographic reconstruction are thesame size. The inventors have recognised that, advantageously, the firstoptical path length and second optical path length may be such that thefirst holographic reconstruction and second holographic reconstructionhave the same resolution. In embodiments, the problem of the mismatch inphysical size and the mismatch in resolution are simultaneouslyaddressed.

The system in accordance with the present disclosure is not obviousbecause historically the Fourier lens are physical lenses and creatingthree physical lenses with precisely the correct focal lengths to causethree different wavelengths to diffract to exactly the same size isgenerally considered impracticable, especially when you consider thatthe focal length tolerance of an average lens is 5%. However, theinventors have recognised that these acceptable practical disadvantagesare out-weighted by the gains in image quality which can be achieved inaccordance with the present subject-matter. In further advantageousembodiments, the Fourier lens is integrated into the hologram as aphase-only lens to make the system further viable because the practicaldisadvantages are yet further out-weighed. In particular, the inventorshave recognised that with pixels small enough, a phase only Fourierlenses of sufficient strength may be made (large pixels and short focallength phase-only lens leads to aliasing and image distortion).

It will be apparent that, in embodiments, the first holographicreconstruction and second holographic reconstruction are coincident.Accordingly, a composite colour holographic reconstruction of apolychromic object may be provided. The first holographic reconstructionrepresents a first wavelength component of an object and the secondholographic reconstruction represents a second wavelength component ofthe object. The present disclosure is not limited to Fourier holographybut, in some embodiments, the first holographic data represents a firstwavelength component of the object in the frequency domain and thesecond holographic data represents a second wavelength component of theobject in the frequency domain.

The Fourier lens may be a phase-only lens forming part of the respectiveholographic data (i.e. hologram). The Fourier lens may be a physicaloptic.

In embodiments, the first Fourier transform lens is a first physicaloptic. In these embodiments in particular, the zero-order removalapertures described may optionally be included on the mirrors andoptical power added to the holographic data such that the correspondingholographic reconstruction and zero-order light are brought to a focusat different planes on the optical path. In other embodiments, the firstFourier transform lens is first lensing data of the first holographicdata. That is, the first holographic data comprises an image componentand a lensing component, wherein the lensing component is the firstFourier transform lens. In embodiments in which the Fourier transformlens is computationally provided on the SLM, it may be understood thatthe first lensing data is software-defined.

Likewise, in embodiments, the second Fourier transform lens is a secondphysical optic. In other embodiments, the second Fourier transform lensis second lensing data of the second holographic data. That is, thesecond holographic data comprises an image component and a lensingcomponent, wherein the lensing component is the second Fourier transformlens. In embodiments in which the Fourier transform lens iscomputationally provided on the SLM, it may be understood that thesecond lensing data is software-defined.

In further advantageous embodiments, the first Fourier transform lens isa physical optic and the second Fourier transform lens is lensing dataof the second holographic data, or vice versa.

The skilled person will readily understand that Equation 2 may beequally applied to different colours/wavelengths to determine differentFourier path lengths in accordance with the present disclosure. Inembodiments, the first wavelength is red light and the second wavelengthis green light. That is, the first wavelength is a wavelength or rangeof wavelengths in the region (or band) of the electromagnetic spectrumcorresponding to red light. Likewise, the second and third wavelengthsare respective wavelengths or ranges of wavelengths in the region (orband) of the electromagnetic spectrum corresponding to green and bluelight, respectively.

It may therefore be understood that, in embodiments, the firstwavelength is greater than the second wavelength and the optical pathlength from the first Fourier transform lens to the replay plane is lessthan the optical path length from the second Fourier transform lens tothe replay plane. In embodiments, the focal length of the Fouriertransform lens is inversely proportional to the wavelength of thecorresponding light.

In embodiments, the SLMs are reflective LCOS SLMs. In other embodiments,the SLMs are transmissive or MEMs based SLMs. In embodiments, the SLMsare phase-modulating only (i.e. not amplitude-modulating). Inembodiments, the first holographic data corresponds to a firstphase-delay distribution and the second holographic data corresponds toa second phase-delay distribution. In other embodiments, the SLMs areamplitude-modulating or amplitude and phase-modulating. That is, inembodiments, the first plurality of pixels are provided by a firstspatial light modulator, optionally, a first reflective LCOS spatiallight modulator and the second plurality of pixels are provided by asecond spatial light modulator, optionally, a second reflective LCOSspatial light modulator. In embodiments, each pixel has a diameter lessthan 15 μm.

In embodiments, a zero-order removal element is provided for one or moreof the colour channels. The zero-order removal element may be considereda spatial filter. Advantageously, removal of the zero-order lightincreases the signal-to-noise ratio of the holographic reconstruction.In embodiments, the system therefore further comprising a first spatialfilter on the optical path from the first Fourier transform lens to thereplay plane arranged to prevent zero-order diffracted light of thefirst wavelength reaching the replay plane and/or a second spatialfilter on the optical path from the second Fourier transform lens to thereplay plane arranged to prevent zero-order diffracted light of thesecond wavelength reaching the replay plane.

In embodiments, the first and/or second spatial filter is/areincorporated on the respective dichroic mirror and comprises a firstportion arranged to provide (or direct) the zero-order diffracted lighton a first optical path and a second portion arranged to provide (ordirect) the higher-order diffracted light on a second optical path. Inembodiments, the first portion is an aperture and the second portion isa reflection (or reflective) portion.

In embodiments, the display system further comprises a first lightsource arranged to illuminate the first plurality of pixels with lightof the first wavelength and a second light source arranged to illuminatethe second plurality of pixels with light of the second wavelength. Inembodiments, the light sources are substantially monochromatic. Inembodiments, the light sources are (spatially) coherent light sourcessuch as lasers.

It may be understood that the present teaching may be extended to morethan two colour channels. In embodiments, red, green and blue colourchannels are provided to produce a colour holographic reconstruction,further colour channels may be added for example red, green, yellow andblue.

In embodiments, the display system therefore further comprises a thirdplurality of pixels arranged to display third holographic datacorresponding to a third holographic reconstruction and receive light ofa third wavelength; a third Fourier transform lens arranged to receivespatially modulated light having a third wavelength from the thirdplurality of pixels and perform an optical Fourier transform of thereceived light to form the third holographic reconstruction at a replayplane, wherein the third holographic reconstruction is formed of lightat the third wavelength, wherein the optical path length from the thirdFourier transform lens to the replay plane is not equal to the opticalpath length from the second Fourier transform lens to the replay planeor the optical path length from the first Fourier transform lens to thereplay plane.

In embodiments, the first holographic reconstruction, second holographicreconstruction and third holographic reconstruction are coincident.Accordingly, a high quality colour holographic reconstruction isachieved.

It may therefore be understand that in embodiments comprising threecolour channels, the first optical path length, second optical pathlength and third optical path length are such that the first holographicreconstruction, second holographic reconstruction and third holographicreconstruction are the same size and have the same resolution. Inembodiments, the third wavelength is blue light.

In embodiments, each holographic reconstruction is one frame of asequence of 2D video frames. In embodiments, the replay plane isspatially remote from a viewer. That is, the replay field is presentedas a virtual image

The display system of the present disclosure may be used to form head updisplays and head mounted displays, holographic projection displaysinter alia. The display system allows for full colour holograms withthe, full resolution of the replay field for each colour.

In some embodiments, the holographic projection system of the presentdisclosure is used to provide an improved head-up display (HUD) orhead-mounted display or near-eye device. In some embodiments, there isprovided a vehicle comprising the holographic projection systeminstalled in the vehicle to provide a HUD. The vehicle may be anautomotive vehicle such as a car, truck, van, lorry, motorcycle, train,airplane, boat, or ship.

The methods and processes described herein may be embodied on acomputer-readable medium. The term “computer-readable medium” includes amedium arranged to store data temporarily or permanently such asrandom-access memory (RAM), read-only memory (ROM), buffer memory, flashmemory, and cache memory. The term “computer-readable medium” shall alsobe taken to include any medium, or combination of multiple media, thatis capable of storing instructions for execution by a machine such thatthe instructions, when executed by one or more processors, cause themachine to perform any one or more of the methodologies describedherein, in whole or in part.

The term “computer-readable medium” also encompasses cloud-based storagesystems. The term “computer-readable medium” includes, but is notlimited to, one or more tangible and non-transitory data repositories(e.g., data volumes) in the example form of a solid-state memory chip,an optical disc, a magnetic disc, or any suitable combination thereof.In some example embodiments, the instructions for execution may becommunicated by a carrier medium. Examples of such a carrier mediuminclude a transient medium (e.g., a propagating signal that communicatesinstructions).

The invention is not restricted to the described embodiments but extendsto the full scope of the appended claims.

1-27. (canceled)
 28. A display system comprising: a first plurality ofpixels arranged to display first holographic data corresponding to afirst holographic reconstruction, to receive light of a first wavelengthfrom a first light source and to spatially modulate the received lightof the first wavelength; a second plurality of pixels arranged todisplay second holographic data corresponding to a second holographicreconstruction, to receive light of a second wavelength from a secondlight source simultaneous with the reception of the light of the firstwavelength by the first plurality of pixels, the second wavelength beingsubstantially different from the first wavelength, and to spatiallymodulate the received light of the second wavelength; a first Fouriertransform lens arranged to perform a Fourier transform of the spatiallymodulated light of the first wavelength to form the first holographicreconstruction at a replay plane, wherein the first holographicreconstruction is formed of light at the first wavelength; a secondFourier transform lens arranged to perform a Fourier transform of thespatially modulated light of the second wavelength to form the secondholographic reconstruction at the replay plane, wherein the secondholographic reconstruction is formed of light at the second wavelength;wherein an optical path length from the first Fourier transform lens tothe replay plane is not equal to an optical path length from the secondFourier transform lens to the replay plane.
 29. The display system ofclaim 28, wherein the first wavelength is greater than the secondwavelength and the optical path length from the first Fourier transformlens to the replay plane is less than the optical path length from thesecond Fourier transform lens to the replay plane.
 30. The displaysystem of claim 28, wherein the focal length of each Fourier transformlens is inversely proportional to the wavelength of the correspondingspatially modulated light.
 31. The display system of claim 28, whereinthe first optical path length and second optical path length are suchthat the first holographic reconstruction and second holographicreconstruction are the same size.
 32. The display system of claim 28,wherein the first optical path length and second optical path length aresuch that the first holographic reconstruction and second holographicreconstruction have the same resolution.
 33. The display system of claim28, wherein the first holographic reconstruction and second holographicreconstruction are coincident.
 34. The display system of claim 28,wherein the first Fourier transform lens is a first physical opticarranged to receive spatially modulated light having the firstwavelength from the first plurality of pixels.
 35. The display system ofclaim 28, wherein the first holographic data comprises an imagecomponent and a lensing component, wherein the lensing component is thefirst Fourier transform lens, the first Fourier transform lens beingsoftware-defined.
 36. The display system of claim 28, wherein the secondFourier transform lens is a second physical optic arranged to receivespatially modulated light having the second wavelength from the secondplurality of pixels.
 37. The display system of claim 28, wherein thesecond holographic data comprises an image component and a lensingcomponent, wherein the lensing component is the second Fourier transformlens, the second Fourier transform lens being software-defined.
 38. Thedisplay system of claim 28, wherein the first holographic reconstructionrepresents a first wavelength component of an object and the secondholographic reconstruction represents a second wavelength component ofthe object.
 39. The display system of claim 28, wherein the firstholographic data corresponds to a first phase-delay distribution and thesecond holographic data corresponds to a second phase-delaydistribution.
 40. The display system of claim 28, further comprising: afirst light source arranged to illuminate the first plurality of pixelswith light of the first wavelength; and a second light source arrangedto illuminate the second plurality of pixels with light of the secondwavelength.
 41. The display system of claim 28, wherein the firstplurality of pixels are provided by a first spatial light modulator,optionally, a first reflective LCOS spatial light modulator and thesecond plurality of pixels are provided by a second spatial lightmodulator, optionally, a second reflective LCOS spatial light modulator.42. The display system of claim 28, further comprising a first spatialfilter on the optical path from the first Fourier transform lens to thereplay plane arranged to prevent zero-order diffracted light of thefirst wavelength reaching the replay plane and/or a second spatialfilter on the optical path from the second Fourier transform lens to thereplay plane arranged to prevent zero-order diffracted light of thesecond wavelength reaching the replay plane
 43. The display system ofclaim 43 wherein the first and/or second spatial filter is a dichroicmirror comprising a first portion arranged to provide the zero-orderdiffracted light on a first optical path and a second portion arrangedto provide the higher-order diffracted light on a second optical path.44. The display system of claim 44 wherein the first portion is anaperture and the second portion is a reflection portion.
 45. The displaysystem of claim 28, further comprising: a third plurality of pixelsarranged to display third holographic data corresponding to a thirdholographic reconstruction and to receive light of a third wavelength; athird Fourier transform lens arranged to perform a Fourier transform ofthe light of the third wavelength to form the third holographicreconstruction at a replay plane, wherein the third holographicreconstruction is formed of light at the third wavelength; wherein anoptical path length from the third Fourier transform lens to the replayplane is not equal to the optical path length from the second Fouriertransform lens to the replay plane or the optical path length from thefirst Fourier transform lens to the replay plane.
 46. The display systemof claim 45 wherein the first holographic reconstruction, secondholographic reconstruction and third holographic reconstruction arecoincident.
 47. The display system of claim 45, wherein the firstoptical path length, second optical path length and third optical pathlength are such that the first holographic reconstruction, secondholographic reconstruction and third holographic reconstruction are thesame size and have the same resolution.
 48. The display system of claim45, wherein the first wavelength corresponds to red light, the secondwavelength corresponds to green light, and the third wavelengthcorresponds to blue light.
 49. The display system according to claim 28,wherein each pixel has a diameter less than 15 μm.
 50. A display systemcomprising: a first plurality of pixels arranged to display firstholographic data corresponding to a first holographic reconstruction, toreceive light of a first wavelength and to spatially modulate thereceived light of the first wavelength; a second plurality of pixelsarranged to display second holographic data corresponding to a secondholographic reconstruction, to receive light of a second wavelength, andto spatially modulate the received light of the second wavelength; afirst Fourier transform lens arranged to perform a Fourier transform ofthe spatially modulated light of the first wavelength to form the firstholographic reconstruction at a replay plane, wherein the firstholographic reconstruction is formed of light at the first wavelength; asecond Fourier transform lens arranged to perform a Fourier transform ofthe spatially modulated light of the second wavelength to form thesecond holographic reconstruction at the replay plane, wherein thesecond holographic reconstruction is formed of light at the secondwavelength; wherein an optical path length from the first Fouriertransform lens to the replay plane is not equal to an optical pathlength from the second Fourier transform lens to the replay plane,wherein the first wavelength is substantially greater than the secondwavelength and the optical path length from the first Fourier transformlens to the replay plane is less than the optical path length from thesecond Fourier transform lens to the replay plane.